Preparation of an aqueous dispersion of copolymer particles of vinyl acetate and a cyclic ketene acetal monomer

ABSTRACT

The present invention relates to a method comprising the steps of a) contacting under emulsion polymerization conditions vinyl acetate; a cyclic ketene acetal monomer; and a monoethylenically unsaturated acid monomer or a salt thereof; to form an aqueous dispersion of copolymer particles comprising structural units of vinyl acetate, the cyclic ketene acetal, and the monoethylenically unsaturated acid monomer or a salt thereof; wherein the cyclic ketene acetal monomer is characterized by following structure: 
                         
where n and the R groups are as defined herein. The method of the present invention provides a water-borne dispersion that is substantially free of volatile organic solvents and that provides a readily degradable polymer that is useful, for example, in the manufacture of biodegradable packaging products.

BACKGROUND OF THE INVENTION

The present invention relates to the preparation of an aqueousdispersion of polymer particles comprising structural units of vinylacetate and a cyclic ketene acetal monomer.

Copolymers of vinyl acetate (VA) and a cyclic ketene acetal (CKA)monomer such as 2-methylene-1,3-dioxepane (MDO) are particularly usefulin applications that require polymer backbone degradation, such asdegradable packaging, or the physical erosion of polymer-based thinfilms and surface coatings. These copolymers are described as beingprepared in the presence of an organic solvent as disclosed, forexample, in Polym. J. 2009, 41, 650-660; Polym. Chem. 2012, 3,1260-1266; Polym. Chem. 2015, 6, 7447-7454; and US 1996/5541275.

Organic solvents are used to prepare CKA-containing polymers becauseCKAs are known to be hydrolytically unstable. For example, MDOhydrolyzes in water to form 4-hydroxybutyl acetate, thereby reducing theefficiency of incorporation of structural units of MDO into thecopolymer backbone. Accordingly, the hydrolytic instability of MDOmandates the use of unreactive organic solvents; unfortunately, thesesolvents are undesirable due to their high cost, the cost associatedwith their removal, recovery, and recycling, and their contribution toremnant volatile organic solvents in the final isolated product.

Accordingly, it would be advantageous to find a way to prepare aqueousbased VA-CKA copolymers in a way that significantly reduces theformation of unwanted hydrolytic byproducts.

SUMMARY OF THE INVENTION

The present invention addresses a need in the art by providing a methodcomprising the steps of a) contacting under emulsion polymerizationconditions at a temperature in the range of from 30° C. to 55° C., andat a pH in the range of from 6.0 to 9.0: 1) 75 to 98.5 parts by weightvinyl acetate; 2) 1 to 20 parts by weight of a cyclic ketene acetalmonomer; and 3) 0.05 to 5 parts by weight of a monoethylenicallyunsaturated acid monomer or a salt thereof; to form an aqueousdispersion of copolymer particles comprising structural units of vinylacetate, the cyclic ketene acetal, and the monoethylenically unsaturatedacid monomer or a salt thereof; wherein the cyclic ketene acetal monomeris characterized by following structure:

-   where n is 0, 1, or 2;-   R is H or C₁-C₆-alkyl;-   R¹ and R² are each independently H, C₁-C₁₂-alkyl, phenyl, or vinyl;    or R¹ and R² together with the carbon atoms to which they are    attached, form a fused benzene ring or a fused C₃-C₇-cycloaliphatic    ring; and-   R^(1′) and R^(2′) is are each independently H or C₁-C₁₂-alkyl; or R¹    and R^(1′) and/or R² and R^(2′) form an exocyclic double bond;-   with the proviso that when n is 1:-   R³ and R^(3′) are each independently H, C₁-C₁₂-alkyl, phenyl, or R³    and R^(3′) form an exocyclic double bond or a spirocycloaliphatic    group or spiro-2-methylene-1,3-dioxepane group;-   with the further proviso that when n is 2:-   each R³ is H, C₁-C₁₂-alkyl, or together with the carbon atoms to    which they are attached form a double bond, a fused benzene ring, or    a fused C₃-C₇-cycloaliphatic ring.

The method of the present invention provides a water-borne dispersionthat is substantially free of volatile organic solvents; the dispersionis useful, for example, in the manufacture of biodegradable packagingproducts.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method comprising the steps of a)contacting under emulsion polymerization conditions at a temperature inthe range of from 30° C. to 55° C., and at a pH in the range of from 6.0to 9.0: 1) 75 to 98.5 parts by weight vinyl acetate; 2) 1 to 20 parts byweight of a cyclic ketene acetal monomer; and 3) 0.05 to 5 parts byweight of a monoethylenically unsaturated acid monomer or a saltthereof; to form an aqueous dispersion of copolymer particles comprisingstructural units of vinyl acetate, the cyclic ketene acetal, and themonoethylenically unsaturated acid monomer or a salt thereof; whereinthe cyclic ketene acetal monomer is characterized by followingstructure:

-   where n is 0, 1, or 2;-   R is H or C₁-C₆-alkyl;-   R¹ and R² are each independently H, C₁-C₁₂-alkyl, phenyl, or vinyl;    or R¹ and R² together with the carbon atoms to which they are    attached, form a fused benzene ring or a fused C₃-C₇-cycloaliphatic    ring; and-   R^(1′) and R^(2′) are each independently H or C₁-C₁₂-alkyl; or R¹    and R^(1′) and/or R² and R^(2′) form an exocyclic double bond;-   with the proviso that when n is 1:-   R³ and R^(3′) are each independently H, C₁-C₁₂-alkyl, phenyl, or R³    and R^(3′) form an exocyclic double bond or a spirocycloaliphatic    group or spiro-2-methylene-1,3-dioxepane group;-   with the further proviso that when n is 2:-   each R³ is H, C₁-C₁₂-alkyl, or together with the carbon atoms to    which they are attached form a double bond, a fused benzene ring, or    a fused C₃-C₇-cycloaliphatic ring.

As used herein, the term “structural unit of vinyl acetate” refers to apolymer backbone containing the following repeat units:

-   wherein the dotted lines represent the points of attachment to the    other structural units in the polymer backbone.

The term “structural unit of a cyclic ketene acetal monomer” is used torefer to a polymer backbone containing the following repeat unit:

-   where R, R¹, R², R³, R^(1′), R^(2′), R^(3′), and n are as previously    defined.

Examples of cyclic ketene acetal monomers include:

A preferred cyclic ketene acetal monomer is 2-methylene-1,3-dioxepane(MDO).

A structural unit of MDO is illustrated:

The ethylenically unsaturated acid monomer can be a carboxylic acidcontaining monomer, a phosphorus acid containing monomer, or a sulfuracid containing monomer, or salts thereof. Examples of suitablecarboxylic acid containing monomers include acrylic acid, methacrylicacid, itaconic acid, crotonic acid, maleic acid, and fumaric acid;examples of suitable phosphorus acid containing monomers includephosphoethyl methacrylate and 2-(methacryloyloxy)ethyl phosphonic acid;suitable sulfur acid containing monomers include2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), vinyl sulfonicacid, 2-sulfoethyl acrylate, 2-sulfoethyl methacrylate, 3-sulfopropylacrylate, 3-sulfopropyl methacrylate, and 2-propene-1-sulfonic acid. Theethylenically unsaturated acid monomer is preferably a sulfonic acidcontaining monomer, preferably the salt of a sulfonic acid containingmonomer, with a salt of 2-acrylamido-2-methylpropane sulfonic acid beingespecially preferred.

Preferably, the polymer particles have a z-average particle size in therange of from 50 nm, more preferably from 80 nm, and most preferablyfrom 100 nm; to preferably 500 nm, more preferably to 300 nm, morepreferably to 200 nm, and most preferably to 150 nm.

Preferably, VA is used in an amount in the range of from 80, morepreferably from 85 parts by weight, to 96.5, more preferably 95 parts byweight based on the weight of the VA, the CKA, and the ethylenicallyunsaturated acid monomer; preferably, the CKA is used in an amount inthe range of from 3 parts by weight to 15, more preferably to 12 parts,and most preferably to 8 weight percent parts by weight based on theweight of the VA, the CKA, and the ethylenically unsaturated acidmonomer; and preferably the monoethylenically unsaturated acid monomeris preferably used in an amount in the range of from 0.1 parts by weightto 3, more preferably to 1 part by weight based on the weight of the VA,the CKA, and the ethylenically unsaturated acid monomer.

The composition preferably comprises a substantial absence of hydrolysisbyproducts of the CKA and VA. More particularly, where the CKA is MDO,the composition preferably comprises less than 20, more preferably lessthan 10, more preferably less than 5, more preferably less than 2, andmost preferably less than 1 weight percent of the undesirable ester4-hydroxybutyl acetate (illustrated below) based on the weight ofstructural units of MDO in the polymer particles.

The composition preferably further comprises less than 3000 ppm, morepreferably less than 2000 ppm of acetic acid, which is an undesiredbyproduct of VA hydrolysis.

In a preferred process of the present invention, VA, the CKA, preferablyMDO, and the monoethylenically unsaturated acid monomer, preferably asalt of a sulfur acid containing monomer, more preferably a salt ofAMPS, are contacted together under emulsion polymerization conditions.Preferably, the VA and the CKA are combined together, then mixed with anaqueous solution of surfactant and the monoethylenically unsaturatedacid monomer under emulsion polymerization conditions. The reaction iscontrolled at a temperature in the range of from 30° C., preferably from35° C., to 55° C., preferably to 50° C., and more preferably to 45° C.,and at a pH in the range of from 6.0, preferably from 6.5, morepreferably from 7.0, and most preferably from 7.5, to 9.0, morepreferably to 8.5. After completion of the polymerization, a dispersionof the copolymer having a solids content preferably in the range of from20, more preferably from 25, and most preferably from 30 weight percent,to 50, and more preferably to 40 weight percent, is obtained.

It has surprisingly been discovered that aqueous dispersions ofcopolymers of VA and CKA can be prepared efficiently without the use oforganic solvents and with a minimum of undesirable hydrolysis byproductsof the CKA and VA.

Method for Measuring Particle Size

Particle size was measured using a Malvern Zetasizer Nano ZS90, whichmeasures Z-average particle size (D_(z)) using dynamic light scattering(DLS) at a scattering angle of 90° using Zetasizer software version7.11. A drop of the sample dispersion was diluted using an aqueoussolution of MilliQ water (18.2 MΩ.cm at 25° C.) to achieve a particlecount in the range of 200-400 thousand counts/s (Kcps). Particle sizemeasurements were carried using instrument's particle sizing method andD_(z) was computed by the software. D_(z) is also known as theintensity-based harmonic mean average particle size and expressed as;

$D_{z} = \frac{\Sigma S_{i}}{\Sigma( {S_{i}/D_{i}} )}$

Here, S_(i) is scattered intensity from particle i with diameter D_(i)Detailed D_(z) calculations are described in ISO 22412:2017 (Particlesize analysis—Dynamic light scattering (DLS)).

Acetic Acid Determination Method

Analysis of acetic acid was performed using a Agilent 1100 Serieshigh-performance liquid chromatography system equipped with a PhenomenexRezex ROA-Organic Acid H+(8%) (250×4.6) mm column (part number00G-0138-E0), Phenomenex Security Guard Carbo-H4 element, UV detectoroperating at a wavelength of 210 nm, and autosampler. The column oventemperature was set to 35° C. and the mobile phase was 2.5 mM phosphoricacid in MilliQ water. The instrument operated at a flow rate of 0.4mL/min (isocratic) and the sample injection volume was 5 μL. Dataacquisition and analysis was performed using Agilent ChemStationsoftware (version B.04.03). Samples were prepared for analysis bydilution in MilliQ water (1:100), followed by agitation on horizontalreciprocal shaker for 10 min. Samples were centrifuged at 100 000 rpmfor 10 min at 25° C., and the supernatant was filtered through a 0.45 μmdisposable syringe filter for injection.

Example 1—Preparation of an Aqueous Dispersion of VA/MDO PolymerParticles

A monomer mixture was prepared in a vessel by combining vinyl acetate(VA, 205.95 g), and 2-methylene-1,3-dioxepane (MDO, 10.88 g).Separately, an aqueous mixture was prepared in a vessel by combiningdeionized water (46.18 g), 2-acrylamido-2-methyl-1-propanesulfonic acidsodium salt (AMPS salt, 1.40 g, 50% active in water), Tergitol-15-S-40secondary alcohol ethoxylate (15-S-40, 1.51 g, 70% active in water) andDisponil® FES-32, sodium lauryl ether sulfonate (FES-32, 6.15 g, 31%active in water). Separately, deionized water (285.0 g) was added to a4-neck 1-L round bottom reactor fitted with an overhead stirrer, acondenser, and thermocouple. The reactor was heated to 40° C., afterwhich time FES-32 (16.4 g, 31% active in water), AMPS salt (3.95 g, 50%active in water), FeSO₄·7H₂O (16.2 g, 0.15 active in water), andethylenediamine tetraacetic acid (EDTA, 3.45 g, 1.0 wt % in water) wereadded to the reactor with the aid of additional deionized water (51.6g). The monomer mixture and the aqueous mixture were fed concurrentlyinto the reactor over 60 min while maintaining the reactor temperatureat 40° C. Concurrently, separately prepared solutions of ammoniumpersulfate/t-butyl hydroperoxide (1.38 g APS and 0.58 g t-BHP in 37.0 gwater) and Bruggolite FF6 (2.62 g in 37.6 g water) were fed into thereactor over 70 min. The reaction was measured to be pH=6.5-7 throughoutthe feed process. Upon completion of addition of the feeds, the reactortemperature was maintained at 40° C. for 15 min, and then cooled to 30°C. The resultant dispersion was adjusted to pH=7 with the addition ofammonium hydroxide (28% active in water) and was filtered through a38-μm screen. The filtrate was analyzed for percent solids (32.9%) andthe z-average particle size was determined to be 116 nm, as measuredusing dynamic light scattering (DLS). The incorporation of2-methylene-1,3-dioxepane (MDO) was measured to be (84.3±4.3)% bydiffusion-edited ¹H NMR spectroscopy. An error of 5% was assumed in allintegrations values and propagated through the calculation for theincorporation of MDO.

Example 2

Example 1 was repeated, but the reaction was maintained at pH=8throughout the feeds by the dropwise addition of ammonium hydroxide (28%active in water). The filtrate was analyzed for percent solids (31.3%)and the z-average particle size was determined to be 122 nm, as measuredDLS. The incorporation of MDO was measured to be virtually quantitative(˜100%) by diffusion-edited ¹H NMR spectroscopy.

Example 3

Example 2 was repeated, but the monomer mixture was prepared bycombining VA (183.95 g), and MDO (32.88 g). The filtrate was analyzedfor percent solids (29.7%) and the z-average particle size wasdetermined to be 92 nm, as measured DLS. The incorporation of MDO wasmeasured to be 97.8±6.6% by diffusion-edited ¹H NMR spectroscopy.

Comparative Example 1

Example 1 was repeated, but the reaction temperature was maintained at60° C. throughout the feeds. The filtrate was analyzed for percentsolids (29.8%) and the z-average particle size was determined to be 177nm, as measured DLS. The incorporation of MDO was measured to be55.1±3.9% by diffusion-edited ¹H NMR spectroscopy.

Comparative Example 2

Example 1 was repeated, but the reaction temperature was maintained at80° C. throughout the feeds. The filtrate was analyzed for percentsolids (29.4%) and the z-average particle size was determined to be 217nm, as measured DLS. The incorporation of MDO was measured to be37.8±2.7% by diffusion-edited ¹H NMR spectroscopy.

Comparative Example 3

Example 1 was repeated, except that the reaction was maintained at pH=5throughout the feeds by the dropwise addition of acetic acid (10% activein water). The filtrate was analyzed for percent solids (31.0%) and thez-average particle size was determined to be 248 nm, as measured DLS.The incorporation of MDO was measured to be 29.0±2.0% bydiffusion-edited ¹H NMR spectroscopy.

Comparative Example 4

Example 1 was repeated, but the reaction was maintained at pH=9.5throughout the feeds by the dropwise addition of ammonium hydroxide (28%active in water). The filtrate was analyzed for percent solids (29.6%)and the z-average particle size was determined to be 283 nm, as measuredDLS. The incorporation of MDO was measured to be 90.3±6.3% bydiffusion-edited ¹H NMR spectroscopy; nevertheless, the sample displayedpoor colloidal stability and 7850 ppm of gel was obtained. The samplediscolored upon standing to a dark brown and an unusually highconcentration of acetic acid and acetaldehyde was observed.

The examples demonstrate the significance of the role that temperatureand pH play in optimizing the incorporation of the water-sensitivemonomers into the polymer backbone and in reducing the production ofunwanted hydrolysis products arising from these monomers.

The invention claimed is:
 1. A method for preparing an aqueousdispersion of vinyl acetate-cyclic ketene acetal copolymer particlescomprising the steps of: a) contacting under emulsion polymerizationconditions at a temperature in the range of from 30° C. to 55° C., andat a pH in the range of from 6.0 to 9.0: 1) 75 to 98.5 parts by weightof vinyl acetate; 2) 1 to 20 parts by weight of a cyclic ketene acetalmonomer; and 3) 0.05 to 5 parts by weight of a monoethylenicallyunsaturated acid monomer or a salt thereof; to form an aqueousdispersion of copolymer particles comprising structural units of vinylacetate, the cyclic ketene acetal, and the monoethylenically unsaturatedacid monomer or a salt thereof; wherein the cyclic ketene acetal monomeris the following structure:

where n is 0, 1, or 2; R is H or C₁-C₆-alkyl; R¹ and R² are eachindependently H, C₁-C₁₂-alkyl, phenyl, or vinyl; or R¹ and R² togetherwith the carbon atoms to which they are attached, form a fused benzenering or a fused C₃-C₇ cycloaliphatic ring; and R¹′ and R²′ are eachindependently H or C₁-C₁₂-alkyl; or R¹ and R¹′ and/or R² and R²′ form anexocyclic double bond; with the proviso that when n is 1: R³ and R³′ areeach independently H, C₁-C₁₂-alkyl, phenyl, or R³ and R³′ form anexocyclic double bond or a spirocycloaliphatic group orspiro-2-methylene-1,3-dioxepane group; with the proviso that when n is2: each R³ is H, C₁-C₂-alkyl, or together with the carbon atoms to whichthey are attached form a double bond, a fused benzene ring, or a fusedC₃-C₇-cycloaliphatic ring.
 2. The method of claim 1 wherein the cyclicketene acetal monomer, the vinyl acetate, and the monoethylenicallyunsaturated acid monomer or a salt thereof are contacted under emulsionpolymerization conditions at a temperature in the range of from 35° C.to 55° C., and at a pH in the range of from 6.5 to 8.5.
 3. The method ofclaim 2 wherein cyclic ketene acetal monomer is selected from the groupconsisting of:


4. The method of claim 3 wherein the cyclic ketene acetal monomer is2-methylene-1,3-dioxepane, and the monoethylenically unsaturated acidmonomer is a sulfur acid containing monomer or a salt thereof; whereinthe 2-methylene-1,3-dioxepane, the vinyl acetate, and the sulfur acidcontaining monomer or a salt thereof are contacted under emulsionpolymerization conditions at a temperature in the range of from 35° C.to 50° C. and at a pH in the range of from 7.5 to 8.5; wherein theaqueous dispersion of polymer particles comprises structural units ofvinyl acetate and 2-methylene-1,3-dioxepane.
 5. The method of claim 4wherein the sulfur acid containing monomer or salt thereof is selectedfrom the group consisting of 2-acrylamido-2-methyl-1-propanesulfonicacid, salts of 2-acrylamido-2-methyl-1-propanesulfonic acid, vinylsulfonic acid, salts of vinyl sulfonic acid, 2-sulfoethyl acrylate,2-sulfoethyl methacrylate, 3-sulfopropyl acrylate, 3-sulfopropylmethacrylate, sodium styrene sulfonate, 2-propene-1-sulfonic acid andsalts of 2-propene-1-sulfonic acid.
 6. The method of claim 5 wherein thesulfur acid containing monomer or salt thereof is a salt of2-acrylamido-2-methyl-1-propanesulfonic acid.
 7. The method of claim 6wherein the aqueous dispersion of polymer particles further comprisesless than 20 weight percent 4-hydroxybutyl acetate based on the weightof structural units of 2-methylene-1,3-dioxepane in the polymerparticles; wherein the 2-methylene-1,3-dioxepane, the vinyl acetate, andthe sulfur acid containing monomer or a salt thereof are contacted underemulsion polymerization conditions at a temperature in the range of from35° C. to 45° C.
 8. The method of claim 4 wherein the aqueous dispersionof polymer particles further comprises less than 10 weight percent4-hydroxybutyl acetate based on the weight of structural units of2-methylene-1,3-dioxepane in the polymer particles.
 9. The method ofclaim 5 wherein the aqueous dispersion of polymer particles furthercomprises less than 2 weight percent 4-hydroxybutyl acetate based on theweight of structural units of 2-methylene-1,3-dioxepane in the polymerparticles.
 10. The method of claim 7 wherein the aqueous dispersion ofpolymer particles further comprises less than 1 weight percent4-hydroxybutyl acetate based on the weight of structural units of2-methylene-1,3-dioxepane in the polymer particles.